In conventional thermal combustion systems, a fuel and air in flammable proportions are contacted with an ignition source, e.g., a spark to ignite the mixture which will then continue to burn. Flammable mixtures of most fuels are normally burned at relatively high temperatures, i.e., in the order of about 3,300.degree. F and above, which inherently results in the formation of substantial emissions of NO.sub.x. In the case of gas turbine combustors, the formation of NO.sub.x can be decreased by limiting the residence time of the combustion products in the combustion zone. However, even under these circumstances undesirable quantities of NO.sub.x are nevertheless produced.
In combustion systems utilizing a catalyst, there is little or no NO.sub.x formed in a system which burns the fuel at relatively low temperatures. Such combustion heretofore has been generally regarded as having limited practicality in providing a source of power as a consequence of the need to employ amounts of catalyst so large as to make a system unduly large and cumbersome. Consequently, combustion utilizing a catalyst has been limited generally to such operations as treating tail gas streams of nitric acid plants, where a catalytic reaction is employed to heat spent process air containing about 2% oxygen at temperatures in the range of about 1,400.degree. F.
In my copending application Ser. No. 358,411, filed May 8, 1973, and incorporated herein by reference, there is disclosed the discovery of catalytically-supported, thermal combustion. According to this method, carbonaceous fuels can be combusted very efficiently at temperatures between about 1,700.degree. and 3,200.degree. F, for example, without the formation of substantial amounts of carbon monoxide or nitrogen oxides by a process designated catalytically-supported, thermal combustion. To summarize briefly what is discussed in greater detail in application Ser. No. 358,411, in conventional thermal combustion of carbonaceous fuels, a flammable mixture of fuel and air or fuel, air, and inert gases is contacted with an ignition source (g.e., a spark) to ignite the mixture. Once ignited, the mixture continues to burn without further support from the ignition souce. Flammable mixtures of carbonaceous fuels normally burn at relatively high temperatures (i.e., normally well above 3,300.degree. F). At these temperatures substantial amounts of nitrogen oxides inevitably form if nitrogen is present, as is always the case when air is the source of oxygen for the combustion reaction. Mixtures of fuel and air or fuel, air, and inert gases which would theoretically burn at temperatures below about 3,300.degree. F are too fuel-lean to support a stable flame and therefore cannot be satisfactorily burned in a conventional thermal combustion system.
In conventional catalytic combustion, on the other hand, the fuel is burned at relatively low temperatures (typically in the range of from a few hundred degrees Fahrenheit to approximately 1,400.degree. F). Prior to the invention described in application Ser. No. 358,411, however, catalytic combustion was regarded as having limited value as a source of thermal energy. In the first place, conventional catalytic combustion proceeds relatively slowly so that impractically large amounts of catalyst would be required to produce enough combustion effluent gases to drive a turbine or to consume the large amounts of fuel required in most large furnace applications. In the second place, the reaction temperatures normally associated with conventional catalytic combustion are too low for efficient transfer of heat for many purposes, for example transfer of heat to water in a steam boiler. Typically, catalytic combustion is also relatively inefficient, so that significant amounts of fuel are incompletely combusted or left uncombusted unless low space velocites in the catalyst are employed.
Catalytic combustion reactions follow the course of the graph shown in FIG. 1 of the accompanying drawing to the extent of regions A through C in that Figure. This graph is a plot of reaction rate as a function of temperature for a given catalyst and set of reaction conditions. At relatively low temperatures (i.e., in region A of FIG. 1) the catalytic reaction rate increases exponentially with temperature. As the temperature is raised further, the reaction rate enters a transition zone (region B in the graph of FIG. 1) in which the rate at which the fuel and oxygen are being transferred to the catalytic surface begins to limit further increases in the reaction rate. As the temperature is raised still further, the reaction rate enters a so-called mass transfer limited zone (region C in the graph of FIG. 1) in which the reactants cannot be transferred to the catalytic surface fast enough to keep up with the catalytic surface reaction and the reaction rate levels off regardless of further temperature increases. In the mass transfer limited zone, the reaction rate cannot be increased by increasing the activity of the catalyst because catalytic activity is not determinative of the reaction rate. Prior to the invention described in application Ser. No. 358,411, the only apparent way to increase the reaction rate in a mass transfer limited reaction was to increase mass transfer. However, this typically requires an increase in the pressure drop across the catalyst and consequently a substantial loss of energy. Sufficient pressure drop may not even be available to provide the desired reaction rate. Of course, more mass transfer can be effected, and hence more energy can always be produced by increasing the amount of catalyst surface. In many applications, however, this results in catalyst configurations of such size and complexity that the cost is prohibitive and the body of the catalyst is unwieldy. For example, in the case of gas turbine engines, the catalytic reactor might very well be larger than the engine itself.
As described in application Ser. No. 358,411, it has been discovered that it is possible to achieve essentially adiabatic combustion in the presence of a catalyst at a reaction rate many times greater than the mass transfer limited rate. In particular, it has been found that if the operating temperature of the catalyst is increased substantially into the mass transfer limited region, the reaction rate again begins to increase rapidly with temperature (region D in the graph of FIG. 1). This is in apparent contradiction of the laws of mass transfer kinetics in catalytic reactions. The phenomenon may be explained by the fact that the temperature of the catalyst surface and the gas layer near the catalyst surface are above the instantaneous auto-ignition temperature of the mixture of fuel, air, and any inert gases (defined herein and in application Ser. No. 358,411 to mean the temperature at which the ignition lag of the mixture entering the catalyst is negligible relative to the residence time in the combustion zone of the mixture undergoing combustion) and at a temperature at which thermal combustion occurs at a rate higher than the catalytic combustion rate. The fuel molecules entering this layer burn spontaneously without transport to the catalyst surface. As combustion progresses and the temperature increases, it is believed that the layer in which thermal combustion occurs becomes deeper. Ultimately, substantially all of the gas in the catalytic region is raised to a temperature at which thermal combustion occurs in virtually the entire gas stream rather than just near the surface of the catalyst. Once this stage is reached within the catalyst, the thermal reaction appears to continue even without further contact of the gas with the catalyst.
The foregoing is offered as a possible explanation only and is not to be construed as in any way limiting the present invention.
Among the unique advantages of the above-described combustion in the presence of a catalyst is the fact that mixtures of fuel and air which are too fuel-lean for ordinary thermal combustion can be burned efficiently. Since the temperature of combustion for a given fuel at any set of conditions (e.g., initial temperature and, to a lesser extent, pressure) is dependent largely on the proportions of fuel, of oxygen available for combustion, and of inert gases in the mixture to be burned, it becomes practical to burn mixtures which are characterized by much lower flame temperatures. In particular, carbonaceous fuels can be burned very efficiently and at thermal reaction rates at temperatures in the range from about 1,700.degree. to about 3,200.degree. F. At these temperatures very little if any nitrogen oxides are formed. In addition, because the combustion as described above is stable over a wide range of mixtures, it is possible to select or control reaction temperature over a correspondingly wide range by selecting or controlling the relative proportions of the gases in the mixture.
The combustion method as described in the copending application Ser. No. 358,411 involves essentially adiabatic combustion of a mixture of fuel and air or fuel, air, and inert gases in the presence of a solid oxidation catalyst operating at a temperature substantially above the instantaneous auto-ignition temperature of the mixture, but below a temperature which would result in any substantial formation of oxides of nitrogen under the conditions existing in the catalyst. The limits of the operating temperaure are governed largely by residence time and pressure. The instantaneous auto-ignition temperature of the mixture is defined above. Essentially adiabatic combustion means in this case that the operating temperaure of the catalyst does not differ by more than about 300.degree. F, more typically no more than about 150.degree. F, from the adiabatic flame temperature of the mixture due to heat losses from the catalyst.
Although the present invention is described herein with particularity to air as the non-fuel component of a fuel-air mixture, it is well understood that oxygen is the required element to support combustion. Where desired, the oxygen content of the non-fuel component can be varied, and the term "air" is used herein to refer to the non-fuel components of the mixtures including any gas or combination of gases containing oxygen available for combustion reactions.
While gas turbine engines employing purely thermal combustion have been used extensively as prime movers, especially in aircraft and stationary power plants, they have not been found to be commercially attractive for propelling land vehicles, such as trucks, buses and passenger cars. One reason for this is the inherent disadvantages of systems based purely on thermal combustion or conventional catalytic combustion. However, with the advance provided in combustion utilizing a catalyst as disclosed and claimed in my said copending application, permitting operation at temperatures of the order of about 1,700.degree. to 3,200.degree. F, such turbine propulsion means for land vehicles and the like now are feasible. However, when employed for propelling land vehicles where frequent shutdowns and intermittent use occur, these systems present substantial difficulties in providing fast and non-polluting start-ups. The use of these turbine systems in land vehicles presents a particular problem in that unless a suitable start-up method is employed, substantial pollution of the atmosphere will result during the time taken to reach full operation of the combustion zone containing a catalyst. Until the catalyst body reaches sufficiently high temperature, large amounts of unburned cabonaceous fuel and carbon monoxide are likely to be discharged into the atmosphere.
It is therefore an object of the present invention to provide an effective method for starting a combustion system utilizing a catalyst, which avoids some or all of these difficulties.